Chlorine dioxide has found wide use as a disinfectant in water treatment/purification, as a bleaching agent in pulp and paper production, and a number of other uses due to its high oxidizing power. There is a variety of chlorine dioxide generator systems and processes available in the marketplace. Most of the very large scale generators employed, for example, in pulp and paper production, utilize an alkali metal chlorate salt, a reducing agent, and an acid in a chemical process for producing chlorine dioxide. These generators and the processes employed also produce by-product salts such as sodium chloride, sodium sulfate, sodium sesquisulfate or sodium bisulfate. In pulp and paper mills, the typical by-product is sodium sulfate (saltcake) which is converted into a sulfur salt of sodium in a high temperature boiler and used in the paper process. Boilers require energy and the paper mills have a limited boiler capacity. Increasing the production of chlorine dioxide generally means increased capital investment to provide the added boiler capacity required to process the added amounts of saltcake by-product produced.
Thus a process which reduces the amount of a by-product salt, such as sodium chloride or sodium sulfate, produced while efficiently generating chlorine dioxide is commercially desirable.
U.S. Pat. No. 3,810,969 issued May 14, 1974 to A. A. Schlumberger teaches a process for producing chloric acid by passing an aqueous solution containing from 0.2 gram mole to 11 gram moles per liter of an alkali metal chlorate such as sodium chlorate through a selected cationic exchange resin at a temperature from 5.degree. to 40.degree. C. The process produces an aqueous solution containing from 0.2 gram mole to about 4.0 gram moles of HClO.sub.3. This process requires the regeneration of the cationic exchange resin with acid to remove the alkali metal ions and the treatment or disposal of the acidic salt solution.
K. L. Hardee et al, in U.S. Pat. No. 4,798,715 issued Jan. 17, 1989, describe a process for chlorine dioxide which electrolyzes a chloric acid solution produced by passing an aqueous solution of an alkali metal chlorate through an ion exchange resin. The electrolyzed solution contains a mixture of chlorine dioxide and chloric acid which is fed to an extractor in which the chlorine dioxide is stripped off. The ion exchange resin is regenerated with hydrochloric acid and an acidic solution of an alkali metal chloride is formed.
In U.S. Pat. No. 4,683,039, Twardowski et al describe a method for producing chlorine dioxide in which the chlorine dioxide is produced in a generator by the reaction of sodium chlorate and hydrochloric acid. After separating chlorine dioxide gas, the remaining sodium chloride solution is fed to a three-compartment cell to form sodium hydroxide and an acidified liquor which is returned to the chlorine dioxide generator.
Each of the above processes produces a fixed amount and type of by-product salt.
M. Lipsztajn et al, teach an electrolytic-dialytic process for producing chloric acid and sodium hydroxide from sodium chlorate. Chlorate ions are transferred through an anion-exchange membrane and sodium ions are Passed through a cation-exchange membrane (U.S. Pat. No. 4,915,927, Apr. 10, 1990).
M. Lipsztajn et al, in PCT application No. WO 92/03374, published Mar. 5, 1992, teach a process for producing a mixture of chloric acid and an alkali metal chlorate by the electrolysis of an alkali metal chlorate solution in the anode compartment of a two-compartment membrane electrolytic cell. A large excess of alkali metal chlorate is present in the mixture to provide a high ratio of alkali metal ions to hydrogen ions. The mixture of chloric acid and an alkali metal chlorate is used to produce chlorine dioxide by chemical or electrochemical reduction.
U.S. Pat. No. 5,122,240 issued Jun. 16, 1992 to G. Cowley et al, describes a process for electrochemically treating a solution containing an alkali metal sulfate and an alkali metal chlorate in the anode compartment of an electrolytic cell. The process produces an acidified solution which is fed to a generator for the production of chlorine dioxide. A high concentration ratio of alkali metal ion to hydrogen ion is maintained. Up to about a 20% reduction in saltcake by product is said to be achieved. The process can employ a plurality of unit cells separated from the adjacent ones by bipolar membranes. A separate electrode rinse solution, such as a sodium sulfate solution is circulated to any additional cells positioned between the cathodic and anodic compartments.